Tunable wavelength filter

Abstract

A tunable wavelength filter comprises a plurality of distributed feedback sections and a phase-control section provided between the distributed feedback sections. The bandgap energy of the phase-control section is greater than that of the distributed feedback sections. In the tunable wavelength filter, electric current injected into the phase-control section is controlled to vary the optical length, so that light having a predetermined wavelength is transmitted through the tunable wavelength filter.

What is claimed is: 
     
       1. A tunable wavelength filter, comprising: a semiconductor substrate having a first electrode provided on a first surface thereof;   first and second distributed feedback sections provided on a second surface of said semiconductor substrate; and   a phase-control section provided on said second surface of said semiconductor substrate to be positioned between said first and second distributed feedback sections, each of said first and second distributed feedback sections and said phase-control section having a second electrode provided on respective surfaces thereof opposite to said semiconductor substrate;   wherein said phase-control section is optically coupled to said first and second distributed feedback sections, and a bandgap energy of said phase-control section is greater than that of said first and second distributed feedback sections.   
     
     
       2. A tunable wavelength filter, according to claim 1, further comprising: first and second films provided on facet planes of said first and second distributed feedback sections opposite to said phase-control section for decreasing reflectivity of said facet planes.   
     
     
       3. A tunable wavelength filter, according to claim 2, wherein: each of said first and second distributed feedback sections comprises:   a diffraction grating of a uniform period provided on said second surface of said semiconductor substrate at portions of said second surface corresponding to said first and second distributed feedback sections;   an optical guiding layer and a buffer layer successively grown on said diffraction grating;   an active layer and a cladding layer successively grown on said buffer layer;   a double channel planar structure grown on said cladding layer; and   wherein said phase-control section comprises:   an optical guiding layer and a buffer layer, provided on said second surface of said semiconductor substrate, which are integral with said optical distributed feedback sections;   a cladding layer having no active layer grown on said buffer layer; and   a double channel planar buried structure grown on said cladding layer.